Method for surface-treatment of zinc metal and zinc alloys

ABSTRACT

A METHOD IS PROVIDED FOR SURFACE TREATMENT OF ZINC METAL AND ZINC ALLOYS TO OBTAIN A PRODUCT OF EXCELLENT CORROSION RESISTANCE, PAINT ADHESION PROPERTIES AND APPEARANCE. ACCORDING TO THIS METHOD, THE ARTICLE HAVING THE SURFACE OF ZINC METAL AND ZINC ALLOYS, IS TREATED WITH AN AQUEOUS SOLUTION OF CHROMIC ACID AND COLLOIDAL SILICA.

ULULLU ubuuuo .L .3111: O

3,556,869 METHOD FOR SURFACE-TREATMENT F ZINC METAL AND ZINC ALLOYS Hideya Okada and Shozo Matsuda, Kawasaki, Misao Ohbu, Kitakyushu, and Hideo Tamura, Kawasaki, Japan, assignors to Nippon Steel Corporation, Tokyo, Japan No Drawing. Continuation-impart of application Ser. No. 438,718, Mar. 10, 1965. This application July 22, 1969, Ser. No. 843,859

Claims priority, application Japan, Mar. 16, 1964, 39/ 14,504 Int. Cl. C23f 7/26 U.S. Cl. 148-61 7 Claims ABSTRACT OF THE DISCLOSURE A method is provided for surface treatment of zinc metal and zinc alloys to obtain a product of excellent corrosion resistance, paint adhesion properties and appearance. According to this method, the article having the surface of zinc metal and zinc alloys, is treated with an aqueous solution Qphromic acid and colloidal silica.

This application is a continuation-in-part of application Ser. No. 438,718, filed Mar. 10, 1965.

FIELD OF THE INVENTION This invention relates in general to a method of the surface-treatment of zinc metal and zinc alloys, and more particularly to a method of chemical surface-treatment of metallic materials having the zinc-covered surface to prevent the formation of white rust on the surface when exposed to high humidity.

Description of the prior art Generally speaking, zinc and zinc alloys are more often used as plating materials in hot dip galvanized articles, zinc electroplating and so on, than they are used as metal and alloys themselves. In both cases, their corrosion resistance is not complete because of the formation of white rust on the surface. This characteristic is a serious disadvantage of zinc and its alloys.

It is said that white rust is liable to grow particularly on the surface of the zinc-plated steel sheet in storage or transportation across the sea, and it grows very often in storage in an environment with high humidity such as in Japan. In order to prevent it, various kinds of treating methods have been investigated. However, a method which is demanded nowadays has to give these materials high corrosion resistance and good paint adhesion property.

So far, chromate treatments consisting of chromic acid or their salts are principally used for prevention of white rust formation, but an adequate procedure has not yet been established.

Special characteristics of zinc-plated steel sheet which are demanded by users, include high corrosion resistance and good paint adhesion after fabrication, as well as before The reason that such demand is raised, is that users require the steel sheet to be excellent in the above proper- BEST AVAlLABLE COPY Patented Jan. 19, 1971 ties even without applying the subsequent chromate treatment by themselves. In addition, since a baking painting has recently been used, the painted material is required to be kept highly corrosion-resistant even after baking.

The above-mentioned is summarized, as follows:

What is demanded for the surface treatment of zincplated steel sheet, are good appearance and high corrosion resistance and good paint adhesion, and also high corrosion-resistance and good paint adhesion after the material is fabricated, such high corrosion resistance to be kept even after baking.

On the other hand, the continuous surface treatment of zinc-plated steel sheet for corrosion resistance, should generally be operated at high speed; therefore, the process of surface-treatment, removing the excess coating with rollers and drying, is preferred to the conventional surface-treating process of products, that is, the process of surface-treatment, water rinsing and drying in many respects. In the former process, there can be used, instead of removing the excess coating with rollers, any method for uniformly applying the surface-treatment solution to the to-be-treated product, such as gas-wiping method; such uniformly solution applying method can be applied to steel sheet of any kind and also any fabricated products of simple shape.

Regarding the surface-treatment of such zinc-plated products as mentioned above, there are issued a considerable number of patents; however, most of them con cern the surface-treating methods of plated products after fabrication, and only a few concern the continuous surface-treating methods for which the process of surfacetreatment, squeezing with rolls, drying can be used.

Widely known among the continuous surface-treating methods is the method developed by Armco Steel Corp., using pure chromic acid, on which a patent was issued (corresponding to Japanese patent publication No. 31/ 5,857) and U.S. Pat. No. 2,665,232 concerning the use of a chromic acid-water glass bath.

However, the methods of these patents are not very effective in making the treated product corrosion-resistive and paint-adhesive after fabrication. Also corrosion resistance after painting is not good in view of the occurrence of the blistering of the paint film after the corrosion tests.

When a chromic acid-water glass bath is used for the treatment solution, the appearance of the treated metal surface is adyegsely affected by Na ions which are contained in the water glass, with increasmg the time after the simple process such as removing the excess coating with rollers and drying.

Accordingly, water rinsing after drying the treated surface is an indispensable step for the one-solution treatment; if water rinsing is omitted, corrosion resistance and paint adhesion of the product will deteriorate. Such deterioration of the corrosion resistance and paint adhesion of the product is considered to be due to the Na ions in the water glass.

If a significant corrosion resistance is required, without rinsing treatment after drying, to the product treated by using the chromic acid-water glass solution, there may be considered the application of the so-called two solution treatment, wherein the article is dipped first in water glass BEST AVAtLABLE cow 3 and then in a chromic acid solution. However, such process is complicated and a loss of the treating solution is very much, and these are the fault of the two-solution treatment.

SUMMARY OF THE INVENTION Having recognized the shortcomings of the conventional surface-treating methods as mentioned above, the present inventors have conducted numerous tests for the continuous treatment of galvanized steel sheet electro-plated zinc steel sheet, and thorough studies on the surface-treating process such as the order of surface treatment, removing the excess coating with rollers, drying and using a chromic acid-colloidal silica bath in comparison with use of a chromic acid bath. Thus they have performed the method of the present invention on the basis of an unexpectedly new idea obtained from the above tests and studies.

An object of the present invention is to provide a simple method for surface-treatment of zinc metal or zinc alloys covering steel sheet, thereby producing each zinc metal and its alloys and steel sheet covered with a highly corrosion-resistive and paint-adhesive coating so as to prevent white rust formation.

Another object of the present invention is to provide a method for surface-treatment of zinc metal and its alloys so as to obtain zinc or zinc alloy-covered steel sheet which is particularly excellent in corrosion resistance and paint adhesion even after fabricated on actual operation conditions.

The other object of the present invention is to provide a method for the production of galvanized sheet steel and electro-plated zinc steel sheet with high corrosion resistance and paint adhesion on a continuous production line, and a method for surface-treatment applicable to also zinc metal and plate per se.

In order to attain the above objects, the present invention comprises the application of a mixture of chromic acid or a solution containing chromic acid as a main component, with colloidal silica, to the surface of zinc metal, zinc alloys and steel sheet covered with them, and the uniform coverage of the surface with said mixture solution by such treatment as removing the excess coating with rollers, and their drying without subsequent water rinsing.

DESCRIPTION OF THE PREFERRED EMBODIMENT An aqueous solution of chromic anhydride or a material composed mainly of the same, with a colloidal silica substance is used for the treating method of the present invention. As such colloidal silica substance is used a solution prepared by removing sodium ions from a solution of NaSiO (water glass) with ion exchange resins or commercial colloidal silica such as Ludox produced by E. I. du Pont de Nemours & Co., Inc., Wilmington, Del., U.S.A. There is also available silicas that are obtained by dispersing gelled SiO uniformly in an alkali solution, or that obtained by the neutralization of water glass with an appropriate kind of acid, containing no Na ions, or that obtained by dispersing solid SiO and that obtained by dialysis of water glass.

At any rate, such Si should be uniformly dispersed to form a colloidal state in the solution. The solution will become more effective, if part of the hexavalent chromium contained in the solution is reduced to trivalent chromium by any method that produces no harmful oxidation products in the solution, such as reduction by using a reducing agent, for example, saccharose, or electrolysis with carbon electrodes.

Regarding the treating solution of the present invention. even if it is of comparatively low concentration, it is effective in making the treated object with good corrosion-resistance and paint-adhesion, but the solution of high concentration is expected to be effective in maintaining such properties even after the product is fabricated.

One embodiment of the treating solution of low concentration of hexavalent chromium ions, contains 0.5 to 10 g./l. metal chromium, added with a colloidal silica substance 1 to g./l. as SiO Even the solution of this concentration range is considerably effective, as mentioned in Examples 1 and 2 below, but its effect in maintaining the above properties of the product after fabrication, is not so high as that obtained with the solution of high concentration.

The above treatment is so carried out that the to-betreated object with the surface of zinc metal or zinc alloys is degreased and rinsed with water to have the surface cleaned, and is then dipped in the treating bath at a room temperature for several to 30 seconds; then the treating solution staying on the surface in excess of the required amount is removed with rubber rollers or any other wipers, and the object is dried.

The use of the above treating solution makes it unnecessary to use water rinsing after drying to make the treated object paint-adhesive. According to the present invention, very good corrosion resistance and paint adhesion of the treated object can be attained even without water rinsing after the applied treating solution is dried.

Moreover, the two-step treatment, that is, the treatment of the object first with colloidal silica only and then with chromic acid, are not so greately different in the above-mentioned effects from the one solution treatment. As a result, there is no need of carrying out drying two times for respective treatments with two kinds of solutions, simplifying the treating process and shortening the drying time.

As for the application to the surface of the object, not only dipping in the solution but also spraying and brushing of the solution can be effected; drying may be made after the solution staying on the surface in excess of the required amount has been removed.

The following are examples of the present invention using the solution of low concentration:

EXAMPLE 1 Chromic anhydride; 5 g./l. Colloidal Silica (as SiO 20 g./l. Water: Remainder Solution temperature: 25 C. Dipping time: 5 seconds EXAMPLE 2 Chromic anhydride: 5 g./l. Saccharose: 1 g./l.

Colloidal silica (as SiO 2O g./l. Water: Remainder Solution temperature: 25 C. Dipping time: 5 seconds The above solution was prepared by heating chromic anhydride and saccharose to react with each other, and then cooling them.

When the galvanized iron sheet treated with the above solution was subjected to the salt water spray test, no white rust was formed in hours after the spraying.

In comparison with this test, the galvanized iron sheet treated with pure chromic acid without addition of col loidal silica was subjected to the same test, which results was that white rust formation started in 10 to 12 hours, and in 24 hours, 20 to 30% of the surface was covered with white rust The coated samples were subjected to Erichsen test for paint adhesion using a paint of melamine alkyd resin series, which result was that when the object had been treated with a chromic acid bath containing colloidal silica, it was excellent in paint adhesion, and proved to be perfect in the tests for paint adhesion.

BEST AVAILABLE COPY The deposited amount of chromium was 0.2 mg./r:n. in each case of Examples 1 and 2, and also about 0.2 mg./m. when no colloidal silica was added.

, The following describes the case in which high concentration of the solution of the present invention was used for treatment.

Zinc metal, zinc alloys and sheet steel covered thereby are applied with a solution consisting of chromic acid and colloidal silica being respectively more than 18 g./l. and more than 50 g./l. (as SiO then subjected to squeezing of the solution staying on the surface in excess of the required amount, preferably with rubber rollers of which the hardness ranged from 40 to 60 degrees measured by the hardness tester of spring type (Japanese Industrial Standard K 4301), under pressure of about 0.520 kg./cm. for uniform application of the solution on the surface, and dried (preferably at temperature between 70 and 250 C. for less than about seconds) without rinsing precedent to the drying.

For uniform coating of the solution, there are available, in addition to the above mentioned method employing rollers, such methods as gas-blow spraying. Particularly the gas-blow spraying is useful, when gas of higher temperature than that needed for drying is used. Thus the drying step can be shortened or omitted.

As for the hot-dipping galvanized sheet steel for the treatment according to the present invention, the sheet steel which is adjusted in the thickness of plating layer with rollers of ordinary type, can be used for this purpose. In this respect, sheets wherein the uniformity of thickness of plating layer is adjusted by gas-blowing are excellent for this purpose. Hot-dipping galvanized sheet steel and zincelectro-plated sheet steel will become products of good properties with a film of about 10 mg./m. to 70 mg./m. (as Cr) per side of the surface formed when these steels are treated continuously at a speed of 50 to 500 ft./min. on the above-mentioned procedures.

When using pure chromic acid only, if its concentration exceeds g./l., the so-treated sheet steel burns wholly yellowish gradually, and at the same time, the uneven deposits of chromic acid which are called the chromic acid stains, start to come about.

If the concentration of chromic acid reaches 18 g./l., the above situation gets worse, seriously defiling the appearance of the finished product. This phenomenon can be found with galvanized iron sheet as well as with zincelectro-plated sheet steel, which cannot be prevented even by controlling the step of squeezing with rollers in the process of surface treatment, squeezing with rollers, drying, nor by changing the kinds of the rollers, but depends only on the concentration of chromic acid.

Therefore, when using pure chromic acid, there is a limit in increasing the deposited amount of chromium for raising corrosion resistance of the treated object. If the amount exceeds that limit, the appearance of product will become non-uniform, degrading the commercial value of product.

The maximum deposited amount of chromium in the case of the treatment in pure chromic acid, solution is less than 10 mg./m. (one side of the surface) on chromium metal basis.

On the contrary, the results with the solutions of chromic acid-colloidal silica series, show that at such a high concentration of chromic acid as 15 g./l., and even as high as 18 g./l. beyond which point a solution of pure chromic acid could not be used, the addition of colloidal silica results in a considerable decrease in chromic acid stains and in the change of color to yellow, as compared with the case in which only pure chromic acid is used.

If the concentration of chromium is raised further, the color changes to yellow uniformly over the surface of the treated object rather than non-uniformly.

Thus, it is considered that colloidal silica is useful for applying the treating solution uniformly on the surface,

and chromic acid is useful for preventing the change of color to yellow.

Therefore, in order to raise the deposited amount of chromium to more than 10 mg./m. (on one side of the surface) all through operations in the plant, there must be used a solution of chromic acid-colloidal silica series, instead of the exclusive use of pure chromic acid.

As mentioned above, the special characteristics of colloidal silica is that it is effective even when applied in a comparatively small amount, and it becomes more effective when concentration ratio between chromic acid and colloidal silica is smaller than A. In this case, the yellow color will again change to the interference color.

As a result of the experiments conducted on the application of a solution of chromic acid-colloidal silica series, it is recognized, as mentiotledaborveftliatcoll'oidal silica has the function to make a treating solution applied uniformly and prevent coloring of the treated object, thus allowing the use of a high concentration of chromic acid in the production of zinc-plated sheet steel of high commercial value.

Further tests were conducted on the structure of the solution of chromic acid-colloidal silica bath, using solutions containing chromic acid at higher concentrations than mentioned in Examples 1 and 2 concerning the use of the solution of low concentration.

As a result, the great improvement in keeping corrosion resistance of the treated object after fabrication, was again confirmed. In reference to the above-mentioned Examples 1 and 2 conducted by using the chromic acid and colloidal silica bath, however, the fabrication of the treated object affected its corrosion-resistance and paintadhesion (though these properties had been much higher before fabrication).

However, use of chromic acid of such a high concentration according to the present invention as more than, say 20 g. per 1., in addition to colloidal silica for, giving commerically good appearance even after processed, is most effective in improving its corrosion resistance at a concentration ratio between chromic acid and colloidal silica of lt more than 3. It is important that acids (such as phosphoric acid ions and other ions containing chlon'ne ions), Na ion and single amines are not present in the colloidal silica. For example, the content of Na should be less than 0.05%. The above concentration ratio of 1:3 meets that for raising discoloring effect.

Even if the concentration of chromic acid is raised, addition of colloidal silica in proportion thereto will ensure the production of zinc-plated steel sheet having an appearance of almost as commercially good as that of the otherwise treated material, that is, an appearance of uniformly yellow color, and also an improved corrosion resistance kept after processing.

Improvement can also be seen with respect to paint adhesion of the treated object after processed, and corrosion resistance after painted. The results of such tests are shown in attached Table 1.

As might be understood from the table, when the concentration of chromic acid is raised very high, it is not necessary to maintain the concentration ratio between chromic acid and colloidal silica of 123 from the standpoints of corrosion resistance and paint adhesion to be kept after the treated object is processed; for example, when the concentration of chromic acid is 35 g./l., 70 g./l. of colloidal silica provides acceptable results. However, when taking into consideration the necessity of high efiiciency of treating operations and good appearance of the treated object, the ratio of nearly 1:3 is preferred.

Embodiments of the optimum composition of the treat ing solution are 20 g. chromic acid and 60 g. colloidal silica; and 35 g. chromic acid and g. colloidal silica. In order to increase the deposited amount of chromium, it is advisable to raise the concentrations of these materials in view of the drying time which is required to be shorter in continuous operations.

was 1 Bosults of Tests On Various Steel Sheet Plated withrZino in llot'Bath for Corrosion Resistanca and Paint Adhesion after Processed and for Corrosion Resistance aftor Painted fgz-gggiggsggsistanca test after the material gilcliigsaogstas't gggoiifgre riiiissgreogial Erickson process kin pass Banding i p in wlling Brim Wot tank Brine w t Brine Du pent Tebbing 2122 2 13 3; tank $52? @1 $22; impact gi g test mt ils/2) 1 2C! 2) process A A A O x O A A x 0 0 o o o 20 o o o o o 35 1 A o A o A 0 o o X 35 0 o o o o o o o o 35 o o oo o o o O o o o o o O 50 170 0 A '70 70 0 Q 0 A 1 100 O O O l A 1 "/0 14 0 O O A 1 70 170 O 0 O A Remarks:

Skin pass: About 15 5!, reduction.

Bending: 180.

Erichson: Below 21 mm from the breakage of the shoot Paint: About 20/4. thick of #36 White Polynzaline (trade-nears) made by Intemational Corporafion Brine spray hast: (According to) Japanese Industrial Standard Z 2371 l-lat Tank: 49C, humidity 100C. Evaluation: Excellent, 0 Good, APoor, X Very p m,

Tno brine spray test and the wet bank test continued respectively for as long as 72 hours and one week, except the rosults misled with X and A, which did not continue so long because rust was produced as early as aft r 24 hours.

Tho nark indicatas that rust was produced 0 n 5 aiter the long-time tests.

What is claimed is:

1. In a process for treating the surface of metals comprising the step of coating the surface of zinc or zinc alloys with a mixed solution consisting essentially of chromic acid and colloidal silica including the step of drying the applied coating solution so as to form a thin chemical film on the so treated surface, the improvement which comprises forming said film by applying to the surface of said article said mixed solution wherein the concentration of chromic acid is between above 10 grams per liter up to 70 grams per liter, as metallic chromium, and the silica concentration, as SiO is between 60 grams per liter and 170 grams per liter.

2. A process according to claim 1 wherein the coated product is passed through the coating solution at a speed of about 50 to 500 feet per minute.

3. A process according to claim 1 wherein the mixed 75 solution of chromic acid and colloidal silica is substan- BEST AVAILABLE COPY tially free of chloride, phosphate and sodium ions and single amines.

4. A process according to claim 1 wherein the ratio of chromic acid as chromium to colloidal silica, as SiO is 1 to more than 3.

5. A process according to claim 1 wherein the concentration of colloidal silica is between 70 and 140 grams per liter.

6. A process according to claim 1 wherein the concentration of chromic acid is metallic chromium exceeds 20 grams per liter.

7. A process according to claim 1 wherein the mixed solution contains more than 18 grams per liter of chromic acid as metallic chromium and more than 50 grams per liter colloidal silica, as SiO References Cited UNITED STATES PATENTS 2,030,601 2/1936 McDonald 1486.16 2,680,081 6/1954 Probert et a1 l17135.1 2,850,415 9/1958 Harrison 1486.2X 2,989,418 6/1961 Harb-augh 117135.1X 3,013,897 12/1961 Capery et al l17135.1X 3,133,829 5/1964 Capery et al 1l7135.1 3,150,015 9/1964 Boyer et al. 1486.2 3,421,949 1/1969 Halversen 148-62 RALPH S. KENDALL, Primary Examiner 

